1. Field of the Invention
This invention relates to a method for the separation and recovery from a hot waste gas of combustion such gaseous components as carbon dioxide in the original hot state without cooling the hot waste gas. The carbon dioxide and other gaseous components thus separated and recovered in the hot state can be utilized for chemical conversion reactions without loss of thermal energy. As a result, the specific gaseous components can be converted with high efficiency into various useful organic substances and such gaseous components as, for example, carbon dioxide which are entrained by the waste gas can be reclaimed at a low cost.
The total amount of energy consumed on the earth is huge and the consumption of this energy entails various environmental problems. One of the most serious of these is global warming and carbon dioxide is the main cause. If the consumption of fossil fuels continues to grow at the present rate, it is estimated that the carbon dioxide discharged by the burning of fossil fuels will by itself raise the average atmospheric temperature of the earth by 1.degree. to 5.degree. C. over the next 30 to 60 years.
For the solution of this problem, it is important to regulate the amount of discharged carbon dioxide. The occurrence of carbon dioxide cannot be eliminated so long as fossil fuels are used as energy sources. It is, therefore, important to separate and recover the carbon dioxide arising from the burning of fossil fuels and utilize it as a useful resources. Conceivable ways of making use of carbon dioxide include reduction to carbon monoxide and conversion into methanol, acetic acid, ethylene glycol, etc.
2. Description of the Prior Art
Known ways of separating and recovering carbon dioxide from mixed gases include the method of gas absorption, the method of low temperature separation (the method of distillation), the method of gas adsorption (the method of pressure swing adsorption and the method of temperature swing adsorption), the method of membrane separation, and the like ("Techniques for Recovery of Carbon Dioxide Gas," p. 22, compiled by Toshinaga Kawai and published by NTS K. K., 1991, and "Atmosphere Enveloping the Earth," p. 115, compiled by Yuji Shindo and published by Ohm K. K., 1993). The method of gas absorption separates carbon dioxide from a mixed gas by contacting the mixed gas at a low temperature with a liquid absorbent capable of selectively absorbing carbon dioxide, thereby effecting selective absorption of the carbon dioxide by the liquid absorbent, and then heating the resultant solution, thereby gasifying the carbon dioxide out of the solution. The method of low temperature separation (the method of distillation) effects the separation of carbon dioxide from a mixed gas by a procedure which comprises compressing and cooling the mixed gas until liquefaction and then subjecting the liquid mixture consequently obtained to fractional distillation. In the method of gas adsorption, the method of pressure swing adsorption (the PSA method) effects the separation of carbon dioxide from a mixed gas by a procedure which comprises pressing the mixed gas into contact with such an adsorbent as, for example, activated carbon or a molecular sieve with micropores, thereby inducing selective adsorption of carbon dioxide, and then reducing the pressure, thereby separating the adsorbed carbon dioxide. This method is implemented, for example, by continuously performing the operations of pressure increase, adsorption, washing, and deaeration in a column. It is highly expensive, however, on account of the cost of the energy needed for increasing the pressure, of the equipment for adsorption and of the adsorbent used for the adsorption.
The method of temperature swing adsorption (the TSA method) attains the .separation of carbon dioxide from a mixed gas by a procedure which comprises causing the mixed gas to contact the same adsorbent as mentioned above at normal room temperature or at a lowered temperature, thereby inducing selective adsorption of carbon dioxide, and then elevating the temperature, thereby separating the adsorbed carbon dioxide. This method, however, is disadvantageous in the points that it incurs a large energy cost at the step of regeneration of the adsorbent owing to the use of steam for this regeneration, entails the time-consuming operation of switching between high temperature and normal room temperature, and requires provision of a large heating device.
The method of membrane separation attains the separation of carbon dioxide from a mixed gas by setting in place in the flow path of the mixed gas a membrane capable of selectively passing carbon dioxide, differentiating the pressure before and after the membrane, and passing the mixed gas through this membrane.
The standard PSA method and TSA method mentioned above are techniques of highly extensive utility for the separation and recovery of specific gaseous components from various kinds of mixed gas. They cannot be easily utilized for the recovery of specific gaseous components from low-grade waste gas of combustion, however, owing to high cost.
The methods heretofore developed for the recovery of specific gaseous components from the waste gas will now be summarized.
A method for recovering valuable components from the waste gas exhausted from a furnace type carbon black production apparatus comprises removing carbon black powders from the waste gas, cooling and exposing to increased pressure the resultant waste gas, then recovering carbon dioxide and carbon monoxide from said gas by absorption with respectively suitable solvents (absorbents), and separating and recovering the hydrogen gas still remaining in the gas by the method of adsorption or the method of low temperature separation (JP-A-57-27,902). This method recovers carbon dioxide and carbon monoxide from the waste gas by absorption with respective liquid absorbents and, particularly for the purpose of the absorption of carbon dioxide, utilizes the popular technique of removing carbon dioxide by the circulation of an alkali solution using an aqueous solution of ethanol amine or an aqueous solution of potassium carbonate as the absorbent.
A method has been proposed for the recovery of carbon dioxide, argon, and nitrogen from combustion waste gas discharged from a low air ratio burner and containing substantially no oxygen by selectively separating carbon dioxide by adsorption at the first pressure swing adsorption step, selectively separating nitrogen or argon by adsorption at the second pressure swing adsorption step, and recovering the remainder of argon or nitrogen (JP-A-63-147,805). This method, however, is characterized by attaining selective adsorptive separation and recovery of carbon dioxide in accordance with the pressure swing adsorption method at an extremely low temperature by means of a carbon molecular sieve, synthetic zeolite, activated alumina, or silica gel as the adsorbent.
Further, for the recovery of specific gaseous components from a mixed gas, a method has been proposed for recovering a specific component such as, for example, carbon dioxide from a combustion waste gas using the method of pressure variation type adsorptive separation (the method of pressure swing adsorption, i.e. the PSA method) (JP-A-01-180,218). This method, however, is characterized by supplying the mixed gas in a form having the specific gaseous component contained therein under increased partial pressures, causing the specific gaseous component to be adsorbed on a clinobutyrolite type adsorbent, separating the adsorbed gaseous component from the adsorbent under a reduced pressure to recover the gaseous component, and performing the foregoing steps cyclically.
Though various methods have been proposed for the separation and recovery of specific gaseous components from waste gas as described above, they invariably carry out the operation of separation after the hot waste gas has been cooled. None of the heretofore known methods performs the operation of separation on a waste gas kept in the initial hot state. Therefore, the specific gaseous components, such as carbon dioxide, cannot be separated hot by any of the methods cited above utilizing currently available technologies.